1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a metal separator in the form of a corrugated plate. The membrane electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. First and second flow fields as passages for any of a fuel gas, an oxygen-containing gas and a coolant are formed on both surfaces of the metal separator.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane (electrolyte). The electrolyte membrane is a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators to form a unit cell for generating electricity. In use, normally, a predetermined number of such unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field is formed in a surface of one separator facing the anode for supplying a fuel gas to the anode, and an oxygen-containing gas flow field is formed in a surface of the other separator facing the cathode for supplying an oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along surfaces of the separators.
In the case where metal separators are used as the separators, by providing grooves as the fuel gas flow field on one surface of the metal separator facing the anode, ridges as the back side of the grooves are formed on the other surface of the metal separator. Further, by forming grooves as the oxygen-containing gas flow field on one surface of the metal separator facing the cathode, ridges as the back side of the grooves are formed on the other surface of the metal separator.
For example, a fuel cell separator is disclosed in Japanese Laid-Open Patent Publication No. 08-222237. As shown in FIG. 17, the fuel cell separator includes a separator plate 1 and a separator frame 2. The separator plate 1 is made of metal, and bosses and dimples are formed on the separator plate 1. Specifically, a large number of projections 3, 4 are formed on the front and back surfaces of the separator plate 1 at intervals of several millimeters. When a fuel cell stack is assembled, the top portions of the projections 3, 4 tightly contact unit cells 5. A fuel gas flow field 6 is formed on the side of the projections 3 between the separator plate 1 and the adjacent unit cells 5. The oxygen-containing gas flow field 7 is formed on the side of the projections 4 between the separator plate 1 and the adjacent unit cells 5.
In the above separator plate 1, the fuel gas flow field 6 is formed by a large number of the projections 3, and the oxygen-containing gas flow field 7 is formed by a large number of the projections 4. Therefore, water produced in the power generation tends to be retained between the projections 3 or between the projections 4. At this time, since the fuel gas and the oxygen-containing gas flows between the projections 3 or the projections 4 around the water produced in the power generation, the water may not be discharged smoothly. Therefore, the flows of the fuel gas and the oxygen-containing gas are inhibited, and the power generation performance is lowered.
The fuel cell stack may adopt the so-called skip cooling structure where each coolant flow field is formed at intervals of a predetermined number of unit cells. For example, each of cell units of the fuel cell may be formed by stacking two membrane electrode assemblies and three metal separators together, and the coolant flow field may be formed in each space between the cell units.
However, in the cell unit, when each of the membrane electrode assembly is held between the projections of the separators on both sides of the membrane electrode assembly, the projections of the adjacent cell units are not in alignment with each other. Therefore, the separators tend to be deformed, and the membrane electrode assembly cannot be held reliably. As a result, the separators and the membrane electrode assembly are damaged, and the fluid does not flow along the membrane electrode assembly smoothly.